1. Field of the Invention
This invention relates to a process for preparing an aqueous polyurethane emulsion and, more particularly, to a process for preparing a homogeneous, stable, anionic, self-emulsified, aqueous, polyurethane emulsion.
2. Description of the Prior Art
There have been known in the art various processes for the preparation of aqueous polyurethane emulsions. For example, a typical prior art process comprises reacting a polyhydroxyl compound with an excess of an organic polyisocyanate so as to form an isocyanate-terminated urethane prepolymer, subjecting the resulting urethane prepolymer to chain-extension using a chain extender having a tertiary amino group thereby to obtain a polyurethane composition having a tertiary amino group, quaternizing the amino group of the resulting composition and thereafter mixing the resulting product with water or an aqueous solution of an acid so as thereby to form a so-called "self-emulsifiable" polyurethane composition which does not contain an emulsifier.
According to another typical conventional process, a polyurethane emulsion is obtained by dispersing a polyurethane composition in water, in the presence of an emulsifier.
Polyurethane resins in general have excellent physical as well as chemical properties. However, the physical and chemical properties of films of the polyurethane resin prepared from the polyurethane emulsions obtained by the abovementioned known processes are far inferior in comparison with cross-linked polyurethane resin films. This is partly because it is not easy to introduce chemical cross-linkages into the polymer and partly because the emulsifier tends to remain in the polyurethane resin.
When a polyurethane polymer has strong cross-linkages, handling of the polymer in the production procedures has generally been difficult, and almost impossible in most cases, due to the poor melting and poor solubility properties of the polymer. In consequence, it is almost impossible to prepare an emulsion by dispersing such a polymer in water. Even if the polyurethane can be dispersed in water, the resulting polyurethane emulsion is extremely unstable.